function displayCitations(){
    let quotes = document.getElementsByTagName("blockquote")
    for(let i=0;i<quotes.length;i++){
        //查找节点
        if(!quotes[i].getAttribute("cite")){
            continue
        }
        let url = quotes[i].getAttribute("cite")
        //获取最后一个元素节点
        let quoteChildren = quotes[i].getElementsByTagName("*")
        console.log('blockquote下所有节点：',quoteChildren)
        if(quoteChildren.length<1){
            continue
        }
        let element = quoteChildren[quoteChildren.length-1]
        //创建链接
        let link = document.createElement("a")
        let link_text = document.createTextNode("source")
        link.appendChild(link_text)
        //给链接定义href属性,值：url
        link.setAttribute("href",url)
        //插入链接
        let superscript = document.createElement("sup")
        superscript.appendChild(link)
        element.appendChild(superscript)
    }
}
addLoadEvent(displayCitations)